'Busy Here'

Hi everybody, I’m randomly getting busy signal and the telephone display says ‘Error code: 486’. When I check the log, this is what I find: ‘Got SIP response 486 “Busy here” back from 10.31.XXX.XXX:5070.

I’ve called the provider and they say they have everything ok at their end. I get this message about 1 out of 5 calls.

What’s happening here?